The invention relates to semiconductor lasers, and more particularly, to high power, single mode semiconductor lasers.
Semiconductor pump lasers are widely used in fiber amplification communication systems such as Dense Wavelength Division Multiplexing Systems (DWDMS). DWDMS typically carry 80 or more channels. The number of channels carried by a communication system is directly proportional to the pump power needed to operate the system. An 80 channel system may require for example, two 980 nm pump lasers and four 1480 nm pump lasers, each operated at 150 mW. By increasing laser pump power, fewer lasers are needed to accommodate all channels, likely reducing system cost and complexity.
Laser pump power may be increased by increasing semiconductor chip length. As chip length increases, gain volume increases thereby increasing power. However, increased chip length may cause internal power loss of the laser""s optical waveguide.
The laser pump power may also be increased by widening the active region of the laser. Resistance decreases as the active region width increases, thereby increasing power. However, increased active region width may result in inclusion of an unwanted second transverse mode. Such multilongitudinal laser mode spectra may adversely affect the performance of fiber optic communication systems, and are therefore undesirable for such applications.
Master Oscillator Power Amplifier (MOPA) structures have also been used to increase power. MOPAs comprise an optical master oscillator followed by a power amplifier stage. MOPA use may not be desirable because MOPA fabrication is complex and coupling light from a MOPA structure into a single mode fiber is difficult.
Therefore, a need exists for a high power, single mode semiconductor laser that is relatively simple to manufacture and is compatible with a fiber amplification communication system.
The invention provides a high power semiconductor laser having a single transverse mode operation. In an exemplary embodiment optical power higher than that generated by conventional pump lasers is achieved by widening the gain medium without inducing the second transverse mode. This is accomplished by providing a small refractive index difference between active and blocking regions of the laser. Illustratively, the refractive index difference between the laser active region material and the laser blocking region material at the fundamental frequency is less than about 0.029. An exemplary refractive index range is about 0.020 to 0.025.